Tunable electromagnetic oscillator using {8 01.4{9 {0 grown linbo{hd 3 {b and method

ABSTRACT

This invention relates to a unique high energy, pulsed, widely tunable, coherent oscillator based on an angle-tuned 1.06 Mu m pumped LiNbO3 parametric oscillator. The oscillator&#39;&#39;s basic 1.4 Mu m to 4.4 Mu m frequency range is extended to the visible and ultraviolet by second harmonic and sum frequency generation in LiNbO3, LiIO3, and KDP. The parametric oscillator source is estimated to be 30% efficient when pumped with a 10 mJ per pulse or 300 mJ per pulse Nd:YAG laser. Similarly, the following mixing and sum generation steps are also shown to be nearly 30% efficient. The parametric oscillator followed by a crystal of AgGaSe2, CdSe, or LiNbO3 thus efficiently tunes over a spectral range between 0.62 Mu m and 27 Mu m. Since all processes are angle phasematched, the tuning rate is rapid.

United States Patent 1 Byer et a1.

[ Nov. 25, 1975 1 TUN ABLE ELECTROMAGNETIC OSCILLATOR USING [01.4] GROWN LiNbO AND METHOD [75] Inventors: Robert L. Byer, Stanford; Richard L. Herbst, Menlo Park, Calif.

[73] Assignee: The Board of Trustees of the Leland Stanford Junior University, Stanford. Calif.

22 Filed: on. 3, 1974 [21] Appl. No.: 511,604

[52] U.S. Cl 307/883; 321/69 R; 331/107 R [51] Int. Cl. H02M 5/04 [58} Field of Search 307/883; 321/69 R;

[56] References Cited UNITED STATES PATENTS 3,665205 5/1972 Bridenbaugh et a1 307/883 Assistant Examiner-Darwin R. Hostctter Primary Examiner-R. V. Rolinec [57] ABSTRACT This invention relates to a unique high energy, pulsed, widely tunable, coherent oscillator based on an angletuned 1.06 pm pumped LiNbO parametric oscillator. The oscillators basic 1.4 pm to 4.4 um frequency range is extended to the visible and ultraviolet by second harmonic and sum frequency generation in UN- bO Li1O and KDP. The parametric oscillator source is estimated to be 30% efficient when pumped with a 10 m1 per pulse or 300 ml per pulse Nd:YAG laser. Similarly, the following mixing and sum generation steps are also shown to be nearly 30% efficient. The parametric oscillator followed by a crystal of Ag GaSe CdSe, or LiNbO thus efficiently tunes over a spectral range between 0.62 pm and 27 um. Since all processes are angle phasematched, the tuning rate is rapid.

7 Claims, 7 Drawing Figures AXIS ETALON 2 TILTED LlNbo 3 BlREFRlNGENT ELEMENT 2 TI LTED AXIS ETALON ELEMENT L06); OUTPUT I5msec l00450mJ i BIREFRINGENT [0L4] M MODE GLAN POLAR IZER -Tcm LASER OUTPUT l5 msec l0 mJ CONFOCAL Focusms 1; Sam LOmJ 0.0IJ O.IJ

PUMP ENERGY Sheet 1 of 3 OUTPUT MIRROR PUMP ETALON MIRROR Nd=YAG KDP POCKELS LASER HEAD o SWITCH Nov. 25, 1975 Nd=YAG AMPLIFIER HEAD OSCILLATOR ZUNING CURVE GLAN POLARIZI:

CW PLATE U.s'. Patent MIRROR MIRROR OPT}? AXIS MIRROR e REFLECTION RANGE LINbO T=|2oc 49 48 47 CRYSTAL ORIENTATION -6 US. Patent Nov. 25, 1975 Sheet 3 of3 3,922,561

25 (CUT FOR POLING) POLING VOLTAGE BOULE 27(F'ABRICATED +V CRYSTAL ROD) [0L3] NORMAL LiNb O M.P. I253C a 5.!5A c B563 TUNABLE ELECTROMAGNETIC OSCILLATOR USING [01.4] GROWN LiNbO AND METHOD The gain bandwidth of the oscillator is near cm". The oscillator can be frequency narrowed by use of two internal etalons or by a birefringent crystal plus an etalon. It is expected that continuous scanning with 1 cm resolution will be possible. For higher resolution, the tuning range is approximately 1 cm, and for single mode operation near 0.1 cm, the oscillator cavity free spectral range.

The combination of wide turning range at high pulse energies is a unique feature of the tunable coherent source described. In addition, the NdzYAG pump laser and all optical nonlinear optical elements of this invention operate at or slightly above room temperature and have no inherent properties that would limit the useful operating life with the exception of the flashlamps used to pump the Nd:YAG laser and amplifier. Based on present lifetime data, the flashlamps last to 10" pulses or over 100 days of continuous operation at 10 pps. Thus, in addition to its unique spectral properties, the system described herein has inherently long operational life with minimum required maintenance.

The oscillator herein uses large, high quality LiNbO boules grown in the [01.4] direction which lies in the yz plane, 38 to the z axis. Following annealing and poling the material is strain free, striation free and of high optical quality. It is also useful as an electro-optic switch, for second harmonic generation as well as tunable parametric generation over the 1.4 p. to 4.4 pm spectral range.

BACKGROUND AND OBJECTS OF THE INVENTION This invention relates to a method and apparatus for providing an extended 0.3 pm 30 pm coherent tunable radiation source. The device is based on a Nd:YAG laser pump source. a singly resonant [01.4] LiNbO, parametric oscillator and mixing in four nonlinear crystals AgGaSe,, CdSe. LiNbO,. It is expected that the device will achieve continuous scanning at 0.1 cm"bandwidth at 10 m1 energy per pulse at 10 pps in the near infrared and visible, and corresponding performance with the energy reduced by the Manley-Rowe factor in the intermediate and far infrared. Spectral bandwidths of the order of 0.01 cm' appear likely for narrow tuning ranges of approximately 1 cm".

The features new achieved that are unique to this invention include: 10 mJ energy per pulse; wide infrared tuning range; use of only one set of mirrors; unique bandwidth control and local tuning methods; rapid wide range tuning; and good frequency stability.

In addition to the foregoing, a new crystal of LiNbO, grown in the [01.4] direction has been successfully grown and finds use not only as disclosed herein, but also as an electrooptic switch and a second harmonic generator.

SUMMARY OF THE INVENTION, BRIEF DESCRIPTION OF THE DRAWINGS. AND DETAILED DESCRIPTION OF THE INVENTION [01.4] LiNbO Crystal Growth LiNbO, has been known and grown as a boule along the [01.0] axis from a congruent composition near a lithium to niobium ratio of 0.48 mole The growth of LINbOg cyrstals from a congruent melt plus improved optical quality tests led to uniform high quality [01.0] axis crystals. These crystals have been successfully used in parametric oscillators pumped by a doubled Q- switch Nd:YAG laser output with a tuning range that extends from 0.6 pm to 3.5 um.

Recently an application for a high energy 1.06 am pumped LiNbO parametric oscillator forced us to reexamine the growth of LiNbO For this oscillator, a LiNbO crystal up to S cm in length at a propagation angle 45 to the optic axis in the negative yz quadrant is required. Although previous 1.06 a parametric oscillators have been demonstrated, they have been limited in output energy due to the short crystals available from [01.0] axis grown boules. Previous growth studies show that LiNbO has another possible growth direction in the [01.4] direction which is 38 to the [00.1] axis in the yz plane.

LiNbO [01.4] boules have now been grown with seeds at the congruent melt composition by the C20- chralski technique. The boules were grown at a growth rate of 15 mm per hour with a 10 rpm crystal rotation rate. Following growth, the boule was slowly cooled at a rate of 50C/hr in an after heater to near room temperature (-1S0 C). It was then transferred to a low temperature gradient furnace and annealed in 0 at a temperature of 1200' C. with an oxygen flow of 2% 1pm. The heating and cooling rates were programmed at 150 C./hr. and 100 C./hr., respectively.

Following annealing the ends of the boule were sawed off and platinum poling electrodes were applied using platinum paste. The boule was poled at 1,200 C. by applying 1.25V/cm poling field along the boule axis for one hour; oxygen flow and heating and cooling rates were the same as those described for annealing. The sign of the poling field was chosen so that the boule axis lie in the negative yz quadrant. This is essential in nonlinear optical applications in order to maximize the nonlinear coefficient. The effective nonlinear coefficient is d =d sin 8 d cos 0 sin 3d) where 6 and da are angles referenced to the z and x axes and d and d are known to be of opposite sign.

Several [01.4] oriented LiNbO, boules have now been grown. To date all of the boules have provided high quality material. Typical boules are 25 mm in diameter by mm in length. The boules have been strain free and of uniformly high optical quality. Fabrication into bars or cylinders has proceeded without difflculty.

We have used [01.4] LiNbO material in an electrooptic modulator and 1.06 p. pumped parametric oscillators. The electro-optic modulator was constructed from three crystals, 2 mm x 2 mm X 25 mm cut with the length along the optic axis. The modulator has operated with a 20 to 1 open to close ratio and has been used for switching a single pulse at 1.06 p. from a mode locked laser pulse train.

Parametric oscillators have been constructed using [01.4] material. The oscillators were directly pumped with a 20 nsec pulse at 1.06 pm from an electro-optic Q-switched Nd:YAG laser operating in the TEM: mode. The measured threshold energy was 4 m1. At 15 m] input, the oscillator had a measured conversion efficiency of 10%. The oscillator operated at room temperature and angle tuned from 1.399 pm to 4.4 pm. A more detailed discussion of this widely tunable oscillator will now be presented.

FIG. 1 is a schematic of the 1.06;). Nd:YAG laseramplifier driver for use in the present invention.

FIG. 2 is a full scale schematic of the angle tuned LiNbO, [L4] singly resonant parametric oscillator constructed in accordance with the present invention.

FIG. 3 is a tuning curve for the LiNbO oscillator vs. crystal angle. The mirror reflection range is indicated.

FIG. 4 is damage limited focal areas vs. input pump energy for LiNbO The plane wave gain approximation applies for input energies greater than 8 m1 at I 5 cm.

FIG. 5 is the spectral range vs. crystal angle for the LiNbO oscillator and following nonlinear crystal harmonic, sum, and mixer generators.

FIG. 6 is the predicted output energy vs. wavelength for the high energy tunable source.

FIG. 7 is a schematic drawing depicting the relationship of the [01.4] LiNbO grown boule of the present invention to the hexagonal crystal structure of LiNbO In general, the tunable oscillator of the present invention comprises a 1.06 m in Nd:YAG laser amplifier driver coupled to the input of an angle tuned LiNbO [01.4] singly resonant parametric oscillator. The pump is a NdzYAG oscillator electro-optically Q-switched using a Kd*P Pockels cell of standard design. The pump oscillator is operated TEM mode and substantially single frequency by proper aperture and etalon control. Nd:YAG lasers meeting this requirement are presently in operation.

The Nd:YAG laser is followed by a double-pass NdzYAG amplifier. The amplifier increases the Nd:YAG laser energy from l0 m] to between I00 and 450 m] depending on the filling factor of the amplifier rod. For this case, the rod diameter is 6 mm and the length is 76 mm. If required, a further increase in energy output can be achieved with a second amplifier rod 1 cm in diameter and 76 mm in length. FIG. 1 shows a schematic of the Nd:YAG pump.

The LiNbO parametric oscillator operates in the singly resonant mode with tuning achieved by crystal rotation. FIG. 2 illustrates the schematic of the oscillator including bandwidth control elements. FIG. 3 shows the tuning curves versus crystal angle for a fixed tem perature near lC.

The LiNbO parametric oscillator is the key element in the chain oftuning elements that follows. Therefore, threshold, conversion efficiency, tuning method and bandwidth have been considered in detail.

For angle-tuned parametric oscillators, the pump beam area must be greater than an area determined by the Poynting vector walk-off and crystal aperture length. Calculations show that for a 2 cm crystal l m] of input pump energy is required to reach maximum gain for off angle phasematched crystals. In this focusing limit. the parametric oscillator gain simplifies to for both 90and off-angle phasematched crystals. Here K (w dln n s c), P is the pump power, I the crystal length and 8 the degeneracy factor. For example, for 5 cm long crystals the gain is maximum at 0.1 m] for l .l/cm damage threshold at 90 phasematching. For 45 phasematching, the gain is maximum for energies greater than 8 m]. At this energy, the gain for off-angle phasematching equals that for 90 phasematching for the limiting input intensity and there is no requirement 4 for the use of 90crystals. This is shown schematically in FIG. 4.

The calculated gain for the parametric oscillator is F 1 0.32 at l MW/cm for a 5 cm crystal. At the MW/cm burn-density limit, the parametric oscillator gain at degeneracy is For efficient operation, gains greater than 4 are adequate. Presently available LiNbO y-axis boules allow crystal lengths up to 2.0 cm. For longer crystals, special boules have been grown in the [01.4] direction and poled to [OLE]. We have recently evaluated a [0LT] grown boule and have shown that quality crystals up to 5 cm in length can be provided at 46phasematching angle. As used herein, [01.1] refers to a direction perpendicular to the 01.4 plane of the crystal and 4 refers to the negative quadrant.

The important conclusion, reached here for the first time, is that parametric oscillator operation at 45 phasematching has the same gain characteristic as phasematched operation for pump energies greater than 8 m]. The electro-optic Q-switched Nd:YAG laser source meets this requirement even without the following amplifier. The amplifier does provide a significant increase in output energy that may improve the usefulness of this source for certain experiments. It is, however, not required for the successful operation of the widely tunable device.

The output of the parametric oscillator tunes between 1.4 um and 4.4 pm using a single set of reflecting optics. This basic frequency range can be extended to cover the 3 pm to 18 um region in AgGaSe, and the ID pm to 27 pm region in CdSe. Various sum generation processes also are possible in LiNbO, and LiIO;. For example, second harmonic generation of the idler in LiNbO covers the 1.06 pm 1.6 pm region, and second harmonic generation of the signal in LiNbO, covers the 0.75 pm 1.06 pm region. These processes angle phasematch and should be -30% efficient. In addition, 1.06 um+idler in LiNbO and 1.06 pm+ signal in LiNbO cover the 0.7 pm 0.8 pm and 0.6 am 0.7 pm spectral range. Because of the high 1.06 pm power available, these steps are also efficient. The above steps all involve a single angle phasematched LiNbO crystal. There may not be any requirement to generate wavelengths shorther than 6000 A; however, by use of a second crystal to sum the oscillator and its second harmonic in Lim the spectral range from 3300 A to 7000 A can also be covered. FIG. 5 illustrates the spectral range versus phasematching angle reached by the LiNbO, parametric oscillator and the following mixers and sum generators.

Conservative predictions of the conversion efficiency of the parametric oscillator and the following mixers and doublers have been made. FIG. 6 shows the expected output pulse energy versus wavelength across the spectral regions covered. For this estimate, we assumed 300 mini pumping energy and only a 30% conversion efficiency in the oscillator. Higher pump energies, up to l J per pulse, are possible in a Nd:YAG amplifier system but at added cost and complexity of an additional amplifier rod. Experimental measurements with the tunable source as a differential absorption transmitter must be made to fully evaluate whether ml per pulse is adequate energy at 10 pps.

The L06 urn Nd:YAG laser pumped LiNbO parametric oscillator forms a nearly ideal primary source for widely tunable 2200 A to 1.5 pm radiation by second harmonic and sum generation, l.4 am to 4.4 pm by parametric oscillation, and 3.0 um to 27 pm by phasematched mixing.

Referring to FIG. 7, the usual hexagonal crystal structure model of LiNbO is depicted by the base 21, and upstanding vertical 2. Superimposed is shown a boule grown along the normal? [01.4] to the 01.4 plane which makes the angle of about 38 to the z axis. Subsequently, the boule is cut at planes 23, 25 and poled by applying the electric field poling voltage across these faces. After poling the fabricated crystal rod 27 is cut from the .boule along the 46 direction indicated by the vector k.

We claim:

1. In an electromagnetic oscillator, means for providing a pump electromagnetic radiation beam having an output at a predetermined frequency, a crystal of UN- b0, grown in the direction perpendicular to [01.4] and poled to [01.4]. means for supporting and orienting said crystal at a phasematching angle to said pump beam and in a path thereof.

2. An electromagnetic oscillator as in claim 1 further including means for varying the orientation, 0, of said crystal to provide a variable output over the range from L4 to 4.4 pm

3. A crystal oscillator as in claim 2 further including means coupled to the output of said crystal for converting the output thereof to other predetermined ranges.

4. An oscillator as in claim 3 in which said conversion means is AgCiase and in which said other range is from 3 to 18 pm.

5. An oscillator as in claim 3 in which said conversion means is CdSe and in which said range is 10 to 27 pm.

6. An oscillator as in claim 3 in which said conversion means is [01.4] LiNbO serving as a second harmonic generator, and in which said range is from 7000 A to 1.4 pm.

7. An oscillator as in claim 3 in which said conversion means is [01.4] LilO serving as a second harmonic generator and in which said range is from 3300 to 7000 A. 

1. IN AN ELECTROMAGNETIC OSCILLATOR, MEANS FOR PROVIDING A PUMP ELECTROMAGNETIC RADIATION BEAM HAVING AN OUTPUT AT A PREDETERMINED FREQUENCY, A CRYSTAL OF LINBO3 GROWN IN THE DIRECTION PERPENDICULAR TO (0.14) AND POLED TO (01.4) MEANS FOR SUPPORTING AND ORIENTING SAID CRYSTAL AT A PHASEMATCHING ANGLE TO SAID PUMP BEAM AND IN A PATH THEREOF.
 2. An electromagnetic oscillator as in claim 1 further including means for varying the orientation, theta , of said crystal to provide a variable output over the range from 1.4 to 4.4 Mu m
 3. A crystal oscillator as in claim 2 further including means coupled to the output of said crystal for converting the output thereof to other predetermined ranges.
 4. An oscillator as in claim 3 in which said conversion means is AgGaSe2 and in which said other range is from 3 to 18 Mu m.
 5. An oscillator as in claim 3 in which said conversion means is CdSe and in which said range is 10 to 27 Mu m.
 6. An oscillator as in claim 3 in which said conversion means is (01.4) LiNbO3 serving as a second harmonic generator, and in which said range is from 7000 A to 1.4 Mu m.
 7. An oscillator as in claim 3 in which said conversion means is (01.4) LiIO3 serving as a second harmonic generator and in which said range is from 3300 to 7000 A. 